Possible chemistry question

The only way the cold water tank is heated is by radiation? No pieces of metal or water lines connecting the two? If so, then yes you have an emissivity problem.

How to fix it, I'm not so sure. Black oxide finish?

George H.

snipped-for-privacy@gmail.com prodded the keyboard with:

A coat of "Blackboard" black will cure that problem. A coat of PVA first will help it stick and not flake with age.

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Best Regards: 
                      Baron.

I am not convinced radiation is the problem.

You've replaced two components, a stove and a tank.

You're focusing on the finish of the tank, but that may be premature.

Is the stove the same dimension? Same distance from the tank? Same thickness metal and runs at the same temperature?

Maybe the tank is anchored different? It was formerly in contact with some common metal bracket, and got most of its heat through conduction?

Same here for almost exactly the same time period. The first few years were hell because I had no clue of what I was doing. I did almost everything wrong.

I'm on my 2nd which has lasted about 30 years: I'm on my 3rd replacement fire brick, 3rd asbestos door seal, and have had to weld a few things back together that had "burnt" out. Note that the door is warped and not making a good seal. I'm debating the merits of straightening the door, replacing the door, or just buying an approved and more efficient stove.

Yeah, but there's almost no thermal mass in a barrel to keep the fire going and to hold the heat inside. Such barrel stoves are very economical to build, but not very economical to operate. If you're going to build your own, get some kind of heavy welded steel box that can be made air tight. Build a labyrinth for the smoke, so that the hot air stays inside the stove longer. Attach an AIR TIGHT door.

The problem with your description and analysis is that it lacks numbers. Some IR temperature measurements would be handy. Also some photos. Personally, a painted black surface is probably best at radiating the heat. That's if you want to heat the room, not keep the heat inside, like a water heater. That's why wood burners are black, and water heaters are white.

I don't think the surface color is going to make much difference. For example, some of the best wood burning stoves from Jotul come in all manner of colors (white, red, green, yellow, etc).

My guess(tm) is something else is happening. However, I can't offer much advice on the something else without a photo and better description. The approximate weight would also be useful. It's no coincidence that the most efficient stoves are monstrous masonry affairs with a huge thermal mass. Also, some clue as to the flue system and how much of it is insulated pipe (Metalbestos or triple wall).

In the few stoves that I've worked on (mostly fixing my own mistakes), the efficiency is mostly the quality of the firewood, the mass of the stove, and a well controlled air flow.

I don't know of any forums or newsgroups that specialize in wood burning water heaters. Incidentally, I've helped build three wood burning hot tubs. They're quite economical to operate but does require some care in siteing, construction, and operation. My favorite mistake was placing the chimney upwind of the tub. It worked well if you wore a gas mask.

Good luck.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

If you think shiney wood burning water heaters don't work, take a look at these made from stainless steel:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Hi Jeff, You want those shinny. (Less heat loss to the surroundings.) I assume there is a water jacket around the stove and water flowing through it from the hot tub.

George H.

I did actually think about taking a belt sander to the new tank but I wasn't too wild about having powdered galvanized material floating around. Dulling the finish would however restore all previous conditions to where they were with the old tank. Lenny

That's also what I assumed. However, it was pointed out that this is not the case. There are two tanks. One has a fire inside. The other is full of water. Nothing but air in between. The distances and measurements are unknown. In effect, it's radiation heating which at the presumed distance, is not terribly efficient.

I still think the stainless hot tub heaters are cool looking (and probably rather expensive). I want one.

The three wood burning hot tub heaters that I built were essentially open air boilers. Basically, a rectangular box made from firebrick. One end has an air tight cast iron stove door. The other has a chimney made from flue pipe. On top was a sheet of copper, steel plate, or stainless steel, depending on the model. 2x12 redwood on top of the metal plate to act as a water tank. A wood grating in the tank to keep the feet from getting scorched. I would build a fire underneath that would heat the metal bottom, which would heat the water in the tank. Temperature was regulated by a nearby cold water hose. The first time I fired it up, I had boiling water in about 30 mins. Oops. If I was in a hurry, I would get some coals going in the wood burner, and then move then quickly to the hot tub heater. All three leaked on the outside (but not into the firebox). I kinda miss it.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I found this picture in the link below of a similar installation. It's not a very good picture but I think you can get the idea. The water tank looks a bit taller than mine, (and it looks to be dull grey). Mine stands 53 inch es and is 15 inches in diameter. The exact size of his tank is hard to tell from the picture. My tank stands right behind the stove and touches it as this one appears to. My system of course is in the basement, and with the e xception that I'm using a barrel for the stove this is essentially what I'm doing. I knew at the time I built this contraption that I could have run c opper coils around the flue or the stove, or inside of the stove for that m atter, add circulating pumps, and perhaps any or all of these might have yi elded a more efficient water heating scenario. However I really wanted to k eep it simple. And it has been and has worked flawlessly all these years.

The other thing is here in Hew Hampshire we frequently lose power. When tha t happens in the Winter it's long and drawn out and most people have to go to a shelter. For us though the transition is pretty seamless. We automatic ally have heat, and we fire up the generator and we can even watch OTA TV i f we want to. So at the expense of a little bit of efficiency I really don' t want to fix, or change the basic concept of this (as they say), "if it ai n't broke".

This latest stove was made out of a heavy gauge 55 gallon galvanized drum.Galvanized was not my choice because it wasn't necessary, but the drum was pristine, the heavy gauge I wanted was available, and the price was right. The finish was flat, and after this heating season most of the galvanized coating has flaked off anyway. So it is essentially a flat finish drum. That seems fine to me.

A little bit about the installation. The output flue pipe comes out of the top rear of the stove. It goes up about four feet and makes a 90 degree turn and travels back about another five feet to where it connects to the wall. About three feet above the stove before I make the 90 degree turn I have installed a damper. I start the stove with the damper o pen, however I'm careful to not run wide open once it gets going as this stove radiates a lot of heat.

There are times, few, but they do sometimes occur (if I'm not careful to close the flue down sufficiently) that certain small areas of the stove may get cherry red. I don't know what temperature that might be, but I'm assuming that it would burn off any paint that might be on it, high temperature rated or not. So for that reason I've never painted any of my stoves.

My first stove was made out of a 55 gallon galvanized hazardous waste container and lasted about ten years. The last stove was made from a

55 gallon stainless steel barrel. It was unpainted and it lasted for almost twenty years. The combination of it and the flat galvanized finish on the old water tank was very successful. So it probably would not be practical for me to paint my current stove, but as it's been suggested here I will paint the water tank.

Another friend to whom I've been talking to about this has suggested that t here are primers available for painting galvanized (or zinc plated) steel. Once primed, I can use any flat black paint on the water tank. He also me ntioned that high temperature paint is only needed on the wood stove drum. "The water tank can use most any flat black paint because the thermal mass of the water tank and the water inside prevents the surface temperature fr om rising more than a couple of degrees above the contents". (This is a quo te). Most of you guys know a lot more about this than I do.

It would be nice to spray this primer and flat paint on to the tank but I c an't because it's indoors I can't control the mist and over spray. So I nee d to start looking around for a suitable primer and flat paint, (both brush on) that would work on galvanized metal.

I really appreciate any further thoughts anyone may have on this. Does anyo ne know of any suitable paint and primer suitable for this application, par ticularly a brush on primer that will adhere to galvanized metal? Thanks, Lenny

I took another look at the picture in the link and that tank looks like an 80 gallon version of mine.

On Wednesday, May 20, 2015 at 9:47:35 AM UTC-4, snipped-for-privacy@gmail.com wrot e: >My tank stands right behind the stove and touches it as this one appears to.

Ah ha!

I really believe (as I have all along) that radiation transfer is a tiny pa rt of the total. Therefore black paint will do little good.

Most of your heat transfer has always occurred through conduction from the contact.

For some reason your new stove and new tank are not making the same contact as before.

There are two things you can do: improve the area of metal to metal contact , and enclose the area between that is not in contact to allow the air to h eat up and give you some convection transfer. I think the most bang for yo ur buck is to improve the contact. Add a brace or two, or even wedge a cur ved piece of metal between.

You might as well paint it first. You fixated on a solution too soon, befo re the root cause was known, and you aren't going to give it up until you'v e tried and failed, at least if you're like most people.

I'd agree that contact is best. If you're convinced that the contact is the same, try an experiment. A no-contact IR thermometer measures radiation. If you use a thermocouple meter to measure the surface temperature and a IR thermometer to measure the same place, the difference in reading is a rough measure on how well radiation will transfer energy to/from the tank. Paint a patch and do the experiment again. This only matters on the side facing the radiation. On the backside, insulate it.

You might also use a small fan to blow air across the stove toward the tank. Or a convection shroud to force the hot air from the stove to pass the tank before entering the room.

This only matters on the side facing the radiation. On the backside, insulate it.

ote:

s to.

part of the total. Therefore black paint will do little good.

e contact.

ct as before.

ct, and enclose the area between that is not in contact to allow the air to heat up and give you some convection transfer. I think the most bang for your buck is to improve the contact. Add a brace or two, or even wedge a c urved piece of metal between.

fore the root cause was known, and you aren't going to give it up until you 've tried and failed, at least if you're like most people.

Hi Tim, The radiation can be a lot more than you expect. If there was a square meter of radiating area, and the stove was ~300F

150 C = 420 K. And the emissivity was one then I calculate a heat transf er of about 1 kW. Radiation goes as the fourth power of the temperature. And ra diation from 300 K to 0K is about 500W/ m^2.

George H.

sfer of

radiation from 300 K to 0K is about 500W/ m^2.

I agree on the 4th power, but I doubt the 300 K delta. I think the stove m ight be 200 F and the tank 100. I base this on water I've put on wood burn ing stoves, it evaporates quickly but doesn't boil. But even if i give you 300 F, you can't tell me the tank is 0 K, that would be -473 F.

Here's a test of contact versus radiation: preheat your oven to 400F and c ook a pizza. Put your hand into the air space and let it absorb heat radia ted from the oven sides and grill surfaces plus heat transfer from the air. Now touch the oven sides or grill surfaces. I think you will find the se cond case more efficient and painful.

I can probably come up with a method of physically "coupling" the back of t he stove to the water tank, to achieve better heat transfer, perhaps with s ome metal in between. I do like that idea however I don't know if that woul d still be OK using non high temp paint on the tank though. Lenny

On Wednesday, May 20, 2015 at 7:54:01 PM UTC-4, snipped-for-privacy@gmail.com wrot e:

the stove to the water tank, to achieve better heat transfer, perhaps with some metal in between. I do like that idea however I don't know if that wo uld still be OK using non high temp paint on the tank though. Lenny

I guess your tank may be cylindrical. Not knowing your actual setup, I'll suggest some dimensions that would need to be site adapted.

Get two lengths of 1 inch copper pipe, about 4 feet long, and press them fi rmly to the gap between tank and stove so that the length of the pipe makes contact with both. One way to do this would be to set each in a 1 quart p lastic bucket filled with concrete on the floor, press them against the tan k, and twist a wire between pipes to tighten them. Or maybe the wire would be enough.

Paint will be an insignificant insulator to heat transfer between metal con tact points.

ansfer of

d radiation from 300 K to 0K is about 500W/ m^2.

might be 200 F and the tank 100. I base this on water I've put on wood bu rning stoves, it evaporates quickly but doesn't boil. But even if i give y ou 300 F, you can't tell me the tank is 0 K, that would be -473 F.

cook a pizza. Put your hand into the air space and let it absorb heat rad iated from the oven sides and grill surfaces plus heat transfer from the ai r. Now touch the oven sides or grill surfaces. I think you will find the second case more efficient and painful.

Hi Tim, So I know P= 500 W/m^2 for k*(300K)^4. (solve for k) I then scaled for T's of 400K to 300K. (Right, maybe the stove is not that hot and the water will heat up and be more than 300 K.. but just to get an idea of the numbers.)

So Power = [(500W/m^2) / 300K^4] * (400^4-300^4) =

500 W/m^2 * 256-81/81 = 500 * 2.16 ~ 1 kW/ m^2.

Your oven example is a bit fallacious. By touching I "cook" the end of my finger very fast.. but how long does it take to heat up my whole arm through my finger?

You have to put in your own numbers for conduction in the stove case. (Mind you I'm not saying you are wrong. It could be that most of the heat is via conduction... you just have to run the numbers.)

George H.

I remember running the numbers for a wall back in engineering school. Heat transfer through a wall is of course a function of the temperature differential (T1-T2) times Area times a coefficient divided by the R value in appropriate units.

It turned out that one nail extending through the wall could transmit a huge amount of heat, even considering the tiny area, because the resistance is so low.

Same with the stove tank problem - one metal connection between them will transmit 100s or 1000s of times the heat of radiation or convection.